You'll want to run "cifs testdc" from the command line of the filer to check that it is on the domain properly and is also happy.
Look at "route -s" to make sure it's not routing traffic via an incorrect network.
Check that your network config matches your switches. So if you have the default MTU of 1500, make sure your switches are okay with this, if you have a VIF configured, make sure the switch config matches this if it's any sort of active-active config, also check that the DNS entries are in and correct, and any search path that needs to be in there is.
It looks like incorrect name resolution is taking place, Have you check the nslookup for the DNS server from the windows machine where you try to open the share. Check the DNS order at the host and keep it matching as per ur order. Sometimes missing SRV records will also create issues.
So the incorrect name resolution may be possible because of wrong manual entry of DNS in the DNS server, or even there could be a stale DNS entry stays in the DNS server which may often need a cache reset @ Server.